1. Field of the Invention
This invention relates to an image formation system such as a copier using electrophotography.
2. Description of the Related Art
Generally, an image formation system using electrophotography forms an electrostatic latent image corresponding to an image signal on a photosensitive body, develops the electrostatic latent image to a toner image, and transfers the toner image to paper, then fixes the toner image on the paper.
Known as this type of image formation system is an image formation system adopting a transport form for feeding paper into an image transfer position of a photosensitive body or an intermediate transfer body by means of a registration roller placed before the transfer position (upstream of paper transport passage).
The image formation system adopting the transport form catches paper transported from the upstream side of the paper transport passage in the space between the registration roller and a pinch roller for pressing against the registration roller and rotates the registration roller in this state, thereby feeding the paper into the image transfer position. At the time, if a gap occurs between the paper arrival timing at the image transfer position and the image arrival timing, the image transfer position on the paper shifts from a desired position; particularly to transfer an image to the full face of paper, margin dimensions become non uniform, etc., leading to extremely poor appearance.
Hitherto, the actual image arrival time at the transfer position, which will be hereinafter referred to as the ideal time, has been calculated based on the image (latent image) write start timing to a photosensitive body. The paper transport speed has been controlled matching the ideal time, whereby the image (toner image) arrival timing at the image transfer position has been matched with the paper arrival timing.
By the way, in recent years, a so-called secondary transfer method, etc., adopting a belt-like body as a photosensitive body or transferring toner images formed on a photosensitive body to an intermediate transfer belt before transferring to paper in batch has been often used for the purposes of improving the transfer efficiency, increasing the processing capability, and the like.
However, in case of using an image carry belt such as a photosensitive belt or an intermediate transfer belt, the belt expands or contracts due to change in temperature, humidity, tension, etc., thus the paper arrival timing at the image transfer position shifts from the ideal time. Since the belt expansion or contraction degree changes depending on the situation, the shift amount from the ideal time also varies. Particularly, to use a highly elastic belt or a belt having a long peripheral length, the shift from the ideal time grows. Further, a slip phenomenon occurs between a belt and a roller for driving the belt; it also becomes a factor for causing shift from the ideal time.